This application is a continuation of PCT application PCT/FR00/02538 filed Sep. 14, 2000 designating the United States, and published in French as WO 01/19765 on Mar. 21, 2001. PCT/FR00/02538 claimed the priority of French application FR 99/11702 filed Sep. 15, 1999. The entire disclosures of both are incorporated herein by reference.
The invention relates to a novel monomer and to the polymer (homopolymer or copolymer) incorporating said monomer, and to their use in organic electroluminescent devices.
Among organic electroluminescent devices, a distinction is made between electroluminescent diodes, also known by the term LED, signifying light emitting diode, and electroluminescent electrochemical cells, also denoted LEC, signifying light emitting electrochemical cell.
LEDs and LECs are well known and correspond to emitters consisting of a thin polymer layer defining a so-called active zone, which is sandwiched between two electrodes, a cathode and an anode respectively, of which at least one is transparent or semitransparent so as to facilitate observation of the luminous emission. The thin film structure of these emitters is particularly advantageous for diffuse lighting and flat screens.
In practice, the semitransparent anode is made of ITO, signifying indium tin oxide, resulting from the doping of In2O3 with SnO2, or of tin oxide doped with antimony or fluorine (SnO2:Sb or SnO2:F), while the cathode is made of aluminum.
The polymers employed within the active zone are so-called conjugated polymers, i.e., polymers whose constituent monomers exhibit an alternation of single bonds, called "sgr" bonds, and double bonds, called xcfx80 bonds, leading to a system of xcfx80 electrons which is highly delocalized along the carbon chain.
Compounds of this kind are derived principally from polyacetylene or else obtained by catenating aromatic nuclei such as benzene, naphthalene, pyrrole, thiophene, pyridine, quinoline, anthracene, carbazole, and fluorene.
In some cases, to allow the steric stresses to be reduced and so to maintain the aromatic entities within the same plane, the abovementioned nuclei are coupled by way of a xe2x80x94(CHxe2x95x90CH)xe2x80x94vinylene bond.
From an electrochemical standpoint, the conjugated polymers can be oxidized or reduced for a structural rearrangement of the alternation of their bonds. More specifically, oxidation, i.e., the loss of an electron, can be interpreted as the making of a hole in the valence band (HOMO), or xe2x80x9cpxe2x80x9d doping of the material. Similarly, reduction, which consists in adding an electron, can be interpreted as being the provision of an electron in the conduction band (LUMO), or xe2x80x9cnxe2x80x9d doping. Consequently, the difference between the oxidation potential and reduction potential of the conjugated polymer can be analyzed as corresponding to the gap of the polymer as a semiconductor material.
Owing to its semiconducting properties, this type of polymer can be employed in the abovementioned organic electroluminescent devices, the phenomenon of electroluminescence resulting from the radiative recombination of an electron from the conduction band and a hole from the valence band, with the carriers (holes and electrons) being injected into the active layer when a potential is applied between the two electrodes.
LEDs and LECs differ in the composition of their active zone.
Accordingly, while the active zone of LEDs consists exclusively of a polymer containing no conductivity additive, which is obtained in film form from a solution in a solvent or by evaporation under vacuum, the layer of the LECs also comprises a salt and a solvent for this salt which is mixed with the conjugated polymer or grafted onto it.
The effect of this difference in composition is to impart different operating voltages to these two devices. Thus with regard to LEDs it is necessary to apply a high operating voltage of more than 10 volts, whereas an operating voltage of between 3 and 4 volts is sufficient for LECs.
The high voltage required for operating LEDs is due, to the very low conductivity of the polymers, which are used in the undoped state, and to the nonohmic nature of the contacts between polymer and electrodes, this nonohmic nature resulting from the existence of potential barriers.
The addition of a salt to the thin layer of the LECs allows the height of this potential barrier to be reduced in accordance with the following phenomenon.
When a low potential is applied between the anode and the cathode, the ions from the dissociated salt migrate toward the electrodes in question to form two fine, charged layers at the interface of the active medium and the electrodes, and these layers will promote the injection of electrons and holes, making the polymer conductive at the polymer/electrode interfaces. When the applied potential exceeds the threshold voltage, the electrons and the holes are injected respectively into the anode/active layer and cathode/active layer interfaces, thereby initiating the formation of a p-n junction. When the voltage is increased again, the additional electrons and holes injected migrate under the effect of this potential excess toward the cathode and the anode. The radiative recombination of these carriers within the space charge zone constitutes the origin of the phenomenon of electroluminescence.
When doping is sufficient, contact between the electrodes and the polymer is ohmic in character, such that the operating voltages are reduced considerably and are close to the theoretical value, viz. the energy difference between the valence band (HOMO) and the conduction band (LUMO) of the polymer.
However, producing the thin layer or active zone of LECs is not simple, on account of the fact that said layer is likely to contain a hydrophobic polymeric material, which is virtually nonpolar, and an ionic species, which undergoes appreciable dissociation only in the presence of a polar solvent.
In order to allow the mixing of the hydrophobic polymer with the hydrophilic salt, it has been proposed that cations and anions resulting from the dissolution of the salt be solvated.
To do this, one first solution consists in solvating the salt, such as the lithium salt, for example, of trifluoromethanesulfonic acid (LiCF3SO3) by means of a solvating polymer of the poly(ethylene oxide) type. However, the two polymers are immiscible, and even less compatible in the presence of salt, so that the mixture obtained is in heterogeneous form.
Another method, which is described in the document WO 97/33326, consists in grafting solvating segments of oligo(ethylene oxide) type onto the frame of a conjugated polymer of the fluorene type. Although this type of compound does make it possible to solve the problem of inhomogeneity of the polymer mixture (phase microseparation), the conjugation which exists between the various monomer units is liable, however, to be modified.
In effect, the solvating segments of oligo(ethylene oxide) type induce a local disorder whichxe2x80x94necessary for ionic conductionxe2x80x94is manifested in a loss of coplanarity of the units which allow the xcfx80 conjugation. In addition, the local disorder is further accentuated by the addition of an ionic compound, the lateral chains being arranged in priority in order to ensure the salvation of the Li+ cations. Moreover, the energy employed by this process (greater than 60 KJ) is markedly greater than the energy gain obtained by virtue of the extension of the xcfx80 conjugation (greater than 10 KJ).
Thus, for example, polythiophenes substituted in position 3 by oligo(ethylene oxide) groups with a mass close to 200 change their optical absorption spectrum by passing from the violet to the yellow in the presence of alkali metal cations, thereby demonstrating the loss of the xcfx80 conjugation.
In parallel, the reduction in conjugation is detrimental to the electronic conductivity, which requires a local order, and to the fluorescence and luminescence.
Furthermore, the reduction in conjugation makes things more difficult for xe2x80x9cnxe2x80x9d and xe2x80x9cpxe2x80x9d doping, which takes place at potentials which are more cathodic and more anodic than the polymers which are not substituted by oligo(ethylene oxide) segments. Consequently, the doped materials are less stable, especially toward agents such as water or atmospheric oxygen.
Finally, the OCH2CH2 groups, corresponding to the direct fixation of the oligo(ethylene oxide) segments on the polymer, have an attractive power which is due to the electronegativity of the oxygen, and which therefore contributes to making xe2x80x9cpxe2x80x9d doping more difficult.
The problem which the invention aims to solve is to provide a monomer having an aromatic nucleus which is equipped with solvating segments, which can be polymerized, and which does not have the disadvantages described above.
The invention accordingly provides a monomer of general formula
Axe2x80x94(Qxe2x80x94S)p,
where:
A=aromatic or heteroaromatic nucleus,
Q=carbon-based or silicon-based divalent radical,
S=aliphatic chain containing at least one polar heteroatom, and
1xe2x89xa6pxe2x89xa66
In other words, the invention consists in having separated the aromatic nucleus A from the monomer of the solvating segment S by the intermediacy of a flexible chemical link Q, thereby allowing spatial separation of the solvating functions from the functions linked to the conjugation. This solution enables the problems set out above to be solved. In effect, the conjugated system is able to adopt a planar conformation which is therefore optimal for its properties of conjugation, doping, conductivity, and luminescence. Independently of any strain on the conjugated system, the solvating segments are able to undergo conformation to provide optimum salvation of the cations and hence better ionic conduction. Since the spacer arm Q is not an electron attracter, it has very little influence on the doping potential, which also makes it possible here to retain stability in the doped polymer.
According to a first feature of the invention, A is selected from the group consisting of the nuclei phenyl, naphthalene, pyrrole, thiophene, benzo-thiophene, pyridine, quinoline, carbazole, anthracene, and fluorene.
In order to improve the conjugation of the system of delocalized xcfx80 electrons, the aromatic or heteroaromatic nucleus A may be engaged in different types of bonds.
This bond may, for example, be in the form of a chain containing 2n atoms, such as C or N, and xe2x80x9cnxe2x80x9d double bonds conjugated with S.
Among these bonds, it is advantageous to employ bonds of the vinylene type (CRxe2x95x90CH)n, where n is between 1 and 5 and R=H, alkyl or aryl containing 1 to 12 carbon atoms. In this case, A is selected from the group consisting of vinylene-phenyl, vinylene-naphthalene, vinylene-pyrrole, vinylene-thiophene, vinylene-benzo-thiophene, vinylene-pyridine, vinylene-quinoline, vinylene-carbazole, vinylene-fluorene, and vinylene-anthracene.
The bond may also be an azomethine bond (CRxe2x95x90N)n or else an azo bond (Nxe2x95x90N)n.
According to another feature of the invention, the monomer, as already stated, has solvating segments S in the form of an aliphatic chain containing at least one polar heteroatom such as O, N, S or P which possesses free electronic doublets capable of interacting with the cations resulting from the dissociation of the salt.
According to a first embodiment, the aliphatic chain S is a polyether of general formula:
xe2x80x94[OCH2CH(R)]rORxe2x80x2xe2x80x94 or xe2x80x94[OCH(R)CH2]rORxe2x80x2
where
r is between 1 and 50
R=H or alkyl containing from 1 to 6 carbon atoms
Rxe2x80x2=alkyl, alkenyl, aryl, arylalkyl or alkenylaryl containing from 1 to 30 carbon atoms.
In a second embodiment, S is a polyamine corresponding to the general formula:
xe2x80x94[N(Rxe2x80x3)CH2CH(R)]rN(Rxe2x80x3)Rxe2x80x2xe2x80x94 or xe2x80x94[OCH(R)CH2]rN(Rxe2x80x3)Rxe2x80x2
in which:
r is between 1 and 50
R and Rxe2x80x3=H or alkyl containing 1 to 6 carbon atoms
Rxe2x80x2=alkyl, alkenyl, aryl, arylalkyl or alkenylaryl containing from 1 to 30 carbon atoms.
In one advantageous embodiment, S is a polyether corresponding to the general formulae above and for which R=Rxe2x80x3=H or CH3 and Rxe2x80x2=methyl, ethyl, butyl, hexyl, octyl, dodecyl, tetradecyl, hexadecyl benzyl.
As already stated, in order to obtain better conductivity of the monomer, the solvating segments are separated from the aromatic nucleus by the intermediacy of flexible bonds.
In a first embodiment, the divalent radical Q constituting the flexible chemical bond corresponds to the general formula (CR1R2)n in which R1 and R2=H, alkyl or alkenyl containing between 1 and 4 carbon atoms and n is between 4 and 24.
The flexible bond Q advantageously has the general formula (CH2)n in which n is between 4 and 24 carbon atoms.
In another embodiment, the divalent radical Q constituting the flexible chemical bond corresponds to the general formula [(Oxe2x80x94Si(R1R2)]n in which R1 and R2=H, alkyl or alkenyl containing between 1 and 4 carbon atoms and n is between 3 and 24.
Furthermore, and according to another characteristic, the monomer of the invention has functions allowing its subsequent polymerization, such as, for example, halogen, amine or aldehyde functions which allow a polycondensation.
In advantageous embodiments, the monomer of the invention is 2,7-dibromo-9,9-bis(7,10,13,16-tetraoxa-heptadecyl)fluorene or 2,7-dibromo-9,9-bis(5,8,11-tri-oxadodecyl)fluorene.
Accordingly, the invention further provides a polymer of homopolymer or copolymer type incorporating the monomer of the invention. This polymer may be random, of random nature, alternating or of alternating nature, block or of block nature.
The methods which allow the coupling and hence the polymerization of the monomers of the invention, either directly or by the intermediacy of the bonds set out above (in particular of the vinyl type), are perfectly well known to the skilled worker.
Mention may be made, for example, of the coupling of monomers bearing two halogen or pseudohalogen groups in the presence of a reducing agent and a catalyst, examples being derivatives of 2,2xe2x80x2-bipyridine or else derivatives of nickel or of palladium complexed with tertiary phosphines.
The monomers having vinylene bonds are obtained by eliminating a sulfonium group obtained from the derivative halogenated in a basic medium in accordance with the following reaction:
xcx9cxcx9cxcx9cAxe2x80x94CH2X+S(R5)2xe2x86x92[xcx9cxcx9cxcx9cAxe2x80x94CH2S(R5)2]++Xxe2x88x92
[xcx9cxcx9cxcx9cAxe2x80x94CH2S(R5)2]++Xxe2x88x92+xe2x88x92X+[S(R5)2xe2x80x94CH2xe2x80x94Axcx9cxcx9cxcx9c]++2OHxe2x88x92xe2x86x92
xcx9cxcx9cxcx9cAxe2x80x94CHxe2x95x90CHxe2x80x94Axcx9cxcx9cxcx9c+2H2O+2S(R5)2+2Xxe2x88x92
in which:
X represents a halogen or a pseudohalogen, Cl, Br, I, SCN, NO2, CF3SO2, or CF3SO3.
R5 represents an alkyl group of 1 to 4 carbon atoms, or else two groups R5 are combined with the sulfur to form a ring, especially when S(R5)2 is tetrahydrothiophene.
xcx9cxcx9cxcx9c represents the growing macromolecular chain.
It is also possible to bring about polymerization by duplicative elimination:
xcx9cxcx9cxcx9cAxe2x80x94CH2X+Xxe2x80x94CH2Axcx9cxcx9cxcx9c+2RtOMxe2x88x92xcx9cxcx9cxcx9cAxe2x80x94CHxe2x95x90CHxe2x80x94Axcx9cxcx9cxcx9c+2XM+2RtOH
in which:
X has the above meanings
Rt is a tertiary organic carbon radical
M is an alkali or alkaline earth metal
Rt is preferably the tert-butyl or tert-amyl group.
The invention likewise provides an organic electroluminescent device consisting of a thin layer comprising the polymer of the invention, said layer being sandwiched between two electrodes.
The electroluminescent polymer may of course be integrated into the active zone of both LEDs and LECS.
In the remainder of the description, however, the polymer of the invention is more particularly used in LECS. In this type of device, the thin layer comprises not only the polymer but also a salt.
Accordingly, and as already stated, the creation of an ohmic junction at the interfaces with the electrodes, and the absence of space charges, result from the formation of doped layers n and p respectively at the negative electrode and the positive electrode.
The polymer of the invention, by virtue of the separation of the solvating segments and the aromatic nuclei, makes it possible to obtain optimum conjugation of the xcfx80 system while respecting the salvation characteristics provided by the groups S.
In practice, the cations resulting from the dissociation of the salt contained within the thin layer constituting the active zone of the LEC are selected from the group consisting of metal cations and omnium type cations. Among the metal cations, a selection is made of the cations derived from rare earths and from transition metals, advantageously the cations derived from alkali metals and from alkaline earth metals.
Moreover, the expression xe2x80x9ccations of onium typexe2x80x9d denotes the cations ammonium (NRi4)+, phosphonium (PRi4)+, amidinium (RC(NRi2)2)+ or guanidinium (C(NRi2)3)+ where R and Ri=H, alkyl or aryl containing 1 to 18 carbon atoms.
The solubility of the salts corresponding to the aforementioned cations, and also the dissociation and conductivity, of course depend heavily on the corresponding anion. Particular preference will be given to anions of low basicity which correspond to the strong acids. Among these, selection will be made in particular of ClO4xe2x80x94, BF4xe2x80x94, PF6xe2x80x94, AsF6xe2x80x94, SbF6xe2x80x94, TeOF5xe2x80x94, RSO3xe2x88x92, (RSO2)2Nxe2x88x92 or (RSO2)2CYxe2x88x92, in which R is a principally perfluorinated radical containing from 1 to 8 carbon atoms and Y=H, CN or RSO2.
According to another feature, the material of the transparent electrode is selected from the group consisting of zinc oxide doped with metallic zinc, gallium, zirconium, tin oxide doped with fluorine or antimony, indium oxide doped with tin, antimony, and fluorine.
As is known, these oxides can be deposited either on layers of glass or a polymer or directly on the electroluminescent material, i.e., the polymer of the invention, via techniques such as cathodic sputtering.
Furthermore, it is also advantageous to select at least one electrode having a high reflective power, consisting preferably of a metal or alloy.
Use is made in particular of a metal selected from the group consisting of aluminum, gold, silver, tin, lead, beryllium, magnesium, calcium, barium, and their alloys.
In order to increase the adhesion or immobilize the active molecules electrochemically or optically, the electrodes may advantageously be surface treated by sizing processes, in particular with organosilanes.
Similarly, in order to modify the injection properties of the carriers (electrons and holes), the electrodes are covered with a layer of an organic or inorganic material.
In one advantageous embodiment, the compound constituting the layer facilitating the injection of carriers is selected from the group consisting of polyquinolines, polycarbazoles, polymers possessing aromatic nuclei linked by cyanovinylene groups C(CN)xe2x95x90CHxe2x80x94, polyanylines, polythiophenes, and especially those possessing phenyl ether substituent functions in position 3 and/or ether substituent functions in position 4, fluorides, such as LiF and MgF2, oxides such as MnO, and nitrides such as AlN or Si3N4.
The derivatives of the monomers of the invention may have various adjuvants added to them. These can be solid particles such as ceramic nanoparticles, oxides such as silica, alumina Al2O3, LiAlO2, ZnO, and SnO2, or nitrides such as AlN. They can also be polymers in the form of latices or fibers, such as polystyrene, polyvinylpyrrolidinone, polyvinylpyridine, polypropylene, and polyethylene. It is also possible to envisage the addition of polyethylene oxides and copolymers thereof, in the presence where appropriate of dispersants, and the addition of plasticizers such as esters of organic polyacids, ethers, and esters of oligo(ethylene oxide).
The invention and the advantages which result from it will emerge more clearly from the following embodiment examples.